Bug 1583813 [wpt PR 19298] - HTML: update <img>.complete tests, a=tes…

…tonly

Automatic update from web-platform-tests
HTML: update <img>.complete tests

Align them somewhat with more modern practices and fix one test that expects img.complete to change during a while loop.

Helps with whatwg/html#1055 and issues pointed out from there.

whatwg/html#4934 has proposed HTML Standard changes.
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wpt-commits: dc2e245a955c832e88d24e6a1af5f718da5491bd
wpt-pr: 19298

UltraBlame original commit: c775395917748fc6bf05f54e385767b974109d4a